In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by an axial-flow compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through an axial-flow turbine. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which in turn provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward.
The various stages of the compressor and the turbine, as well as a turbofan if present, are mounted upon and linked together by shafts and shaft segments extending along the centerline of the gas turbine engine. Some of the shafts are made of high-strength steels. These shafts must have good strength, but equally importantly they must have good low-cycle-fatigue lives in torsion because of the types of loadings imposed upon the shafts.
Traditionally, the shafts had been made of maraging steels, which contain titanium nitride precipitates. After studies showed that these precipitates limit the low-cycle torsional fatigue lives, a family of high-strength, low-titanium maraging steels was developed. These steels are strengthened by aluminum additions on the order of from about 0.5 to about 1.3 weight percent, which replace the titanium additions of the earlier generation of steels. These higher-aluminum steels are described in U.S. Pat. No. 5,393,488, whose disclosure is incorporated by reference. The steels of the ""488 patent result in significantly improved fatigue lives in the shafts.
However, an opportunity for improvement remains. There is an ongoing need to further increase the fatigue lives of the steels of the ""488 patent, without adversely affecting the strength, toughness, and other properties of the steels, and without requiring major alterations to the processing parameters. The present invention fulfills this need, and further provides related advantages.
The present invention provides an improved melting and casting practice for high-aluminum steels such as those of the ""488 patent. The new approach reduces the presence of inclusion clusters based on aluminum-oxygen compositions. Such clusters, when present, may serve as sites for the initiation of fatigue failure. The other desirable mechanical properties of the steels are not adversely affected by the practice of the present invention. The steels produced by the present approach find use as shafts for gas turbine engines and in other applications as well.
A method for fabricating a steel article comprises the steps of providing an iron-base alloy having less than about 0.5 weight percent aluminum, thereafter melting the alloy to form a melt, thereafter adding a first deoxidizer (preferably calcium) addition to the melt, thereafter adding aluminum to the melt to increase the aluminum content of the melt to more than about 0.5 weight percent aluminum, and thereafter casting the melt to form a casting.
The iron-base alloy initially provided desirably has less than about 0.5 weight percent aluminum, and preferably less than about 0.1 weight percent aluminum in order to use the preferred melting practice. In an embodiment, the as-provided iron-base alloy has from about 10 to about 18 weight percent nickel, from about 8 to about 16 weight percent cobalt, from about 1 to about 5 weight percent molybdenum, less than about 0.5 (and preferably less than about 0.1) weight percent aluminum, and from about 1 to about 3 weight percent chromium, balance iron and minor amounts of other elements. The aluminum addition desirably increases the aluminum content of the melt to from about 0.5 to about 1.3 weight percent aluminum. In an embodiment, the final casting desirably has from about 10 to about 18 weight percent nickel, from about 8 to about 16 weight percent cobalt, from about 1 to about 5 weight percent molybdenum, from about 0.5 to about 1.3 weight percent aluminum, from about 1 to about 3 weight percent chromium, up to about 0.3 weight percent carbon, less than about 0.1 weight percent titanium, balance iron and minor amounts of other elements.
In the usual case, the initially provided iron-base alloy has a relatively high carbon content, usually more than about 0.3 weight percent. It is preferred to melt the initially provided iron-base alloy in a vacuum furnace, gradually reducing the pressure while a carbon-oxygen chemical reaction (termed a carbon boil) occurs to reduce the oxygen content of the melt to less than about 10 parts per million by weight. The first calcium addition is then made, preferably in an amount of more than about 200 parts per million by weight. Optionally but preferably, there is an additional step, performed concurrently with the step of adding aluminum, of adding a second calcium addition to the melt, desirably in an amount of from about 100 to about 200 parts per million by weight. Optionally but preferably, there is an additional step, after the step of adding aluminum and before the step of casting, of adding a third calcium addition to the melt, desirably in an amount of from about 50 to about 150 parts per million by weight. Calcium additions are preferably made in alloy form, such as NiCa. The calcium additions deoxidize the melt during the period when aluminum-oxygen-based clustered inclusions would otherwise form, reducing the incidence of the formation of such clustered inclusions that, if present, compromise the low-cycle-fatigue performance of articles made of the steel.
The present steels are typically not used in an as-cast state, but are normally mechanically worked (including mechanical working and/or thermomechanical processing). In the application of most interest, the casting is mechanically worked to form a shaft of a gas turbine engine.
In a preferred embodiment, a method for fabricating a steel article comprises the steps of providing an iron-base alloy having more than about 0.3 weight percent carbon and less than about 0.1 weight percent aluminum, and thereafter melting the alloy in a vacuum furnace to form a melt. The step of melting the alloy includes gradually reducing the pressure within the vacuum furnace to induce a carbon boil in the melt, which reduces the oxygen content of the melt to less than about 10 parts per million by weight. A first addition of calcium is thereafter added to the melt in an amount of more than about 200 parts per million by weight. The method further includes thereafter simultaneously adding aluminum to the melt to increase the aluminum content of the melt to more than about 0.5 weight percent aluminum, and adding a second calcium addition to the melt in an amount of from about 100 to about 200 parts per million by weight. A third calcium addition is thereafter preferably made to the melt, preferably in an amount of from about 50 to about 150 parts per million by weight. The indicated amounts of the calcium additions are for typical cases. The amounts of the additions may be varied as necessary responsive to the amount of oxygen actually present in the melt, which may be readily measured by conventional real-time techniques. Other operable chemical oxidizers may be substituted for the calcium. The melt is thereafter cast, and the casting is mechanically worked. Consistent features discussed elsewhere herein are applicable to this embodiment.
In another embodiment, a method for fabricating a steel article comprises the steps of melting an iron-base alloy having less than about 0.5 weight percent aluminum while reducing the oxygen content of the melt to less than about 10 parts per million by weight. The step of reducing the oxygen content includes the step of adding a deoxidizer, such as calcium, to the melt. Aluminum is added to the melt to increase the aluminum content of the melt to more than about 0.5 weight percent aluminum; and thereafter the melt is cast to form a casting. Preferably, the melt initially has less than about 0.1 weight percent aluminum and more than about 0.3 weight percent carbon. Consistent features discussed elsewhere herein are applicable to this embodiment.
The compositions of the ""488 patent achieved major improvements to the low-cycle-fatigue life of the steel by reducing the titanium content of the steel, thereby reducing the presence of titanium nitride inclusions. These inclusions were observed to be a source of the initiation of fatigue failures. The steels of the ""488 patent are strengthened by the addition of aluminum in what are relatively large amounts for steels, on the order of 0.5-1.3 weight percent. The present inventors observed that fatigue failures in cast-and-worked final articles made of this and similar high-aluminum steels may initiate at aluminum-oxygen-based clustered inclusions (sometimes termed xe2x80x9craftsxe2x80x9d). These aluminum-oxygen-based clusters have been traced back to the melting practice. When the high-aluminum steel alloy is melted prior to casting, the aluminum may form the aluminum-oxygen-based inclusion clusters in the molten steel. These inclusion clusters persist into the casting and then into the mechanically worked final product, leading to premature fatigue failure.
One potential approach to alleviating this problem is to add calcium to the high-aluminum steel melt immediately prior to casting. However, the present studies showed that the addition of calcium to the high-aluminum melt immediately before casting was not sufficient to avoid the presence of the aluminum-oxygen-based inclusions in the final product.
Instead, it has been found that the inclusion problem may be significantly reduced by first preparing the melt with a relatively low aluminum content, and then adding calcium prior to the addition of the remaining aluminum to bring the aluminum content to that desired in the final product, typically from about 0.5 to about 1.3 weight percent. Calcium is optionally but preferably added simultaneously with the aluminum addition as well. The elevated calcium content in the melt reduces the free oxygen available to form aluminum-oxygen based clusters. The calcium reacts with the free oxygen in the melt to form products wherein the oxygen is no longer free, such as calcium oxide and/or calcium aluminate. Further calcium may optionally be added after the aluminum is added to react with oxygen that may be introduced into the melt during the processing of the melt prior to casting. There is a reduced concentration of aluminum-oxygen-based clusters in the final product. Deoxidizers that are functionally equivalent to calcium may be used as well.
The result of this practice change is an improved low-cycle-fatigue life in the final articles produced from the steel. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.